1. Field of the Invention
The present invention relates to an improvement in an electric power steering apparatus carried on a vehicle.
2. Description of the Related Art
In recent years, electric power steering apparatuses have been widely used to provide a comfortable steering feel to a driver by reducing the manual steering effort or force needed to turn a steering wheel. These electric power steering apparatuses are constructed such that an auxiliary torque generated by an electric motor in proportion to a steering torque is transmitted to a steering system. One example of such electric power steering apparatuses is disclosed in Japanese Patent Laid-Open Publication No. HEI 7-165089 entitled "Electric Power Steering Apparatus".
The electric power steering apparatus disclosed in Japanese Patent Laid-Open Publication No. HEI 7-165089 includes a rack shaft having a ball nut mechanism assembled with one end portion thereof, a rack provided at the other end portion thereof, and a pinion meshing with the rack. The electric power steering apparatus also includes a rack guide for urging the rack shaft against the pinion, which rack guide is disposed at a position where the pinion meshes with the rack, thereby eliminating a play at the meshed portion.
Reference is now made to (a) to (d) of FIG. 8 hereof, which diagrammatically illustrate the electric power steering apparatus disclosed in Japanese Patent Laid-Open Publication No. HEI 7-165089.
(a) of FIG. 8 diagrammatically illustrates in top plan the electric power steering apparatus. As shown in this figure, the steering apparatus 100 comprises a rack shaft 101 with steered wheels 104, 104 connected to opposite ends thereof via tie rods 102, 102 and knuckle arms 103, 103. A ball nut mechanism 105 is assembled at one end of the rack shaft 101. A rack 106 is provided at the other end of the rack shaft 101. A pinion 107 is in meshing engagement with the rack 106. The rack shaft 101 is pressed against the pinion 107 by a rack guide 108 located oppositely from the pinion 107.
(b) of FIG. 8 is a separate diagrammatic illustration of the rack shaft 101 and associated members shown in (a) of FIG. 8. As shown in this figure, the rack shaft 101 is supported at two different points, namely, at a position where the ball nut mechanism 105 is disposed and at a position where the pinion 107 and the rack guide 108 are provided. Thus, the rack shaft 101 has a long support span, that is, it is supported at two different points of support spaced apart a long distance.
(c) and (d) of FIG. 8 diagrammatically illustrate operations of the rack shaft 101 and associated members shown in (b) of FIG. 8. As can be seen from these figures, a road surface reactive force is transmitted to the rack shaft 101 via the tie rods 102, 102 during running of a vehicle, particularly upon turning of a steering wheel. Thus, an external force or moment (hereinafter called "moment M") resulting from the road surface reactive force is applied to the opposite ends of the rack shaft 101. Consequently, the rack shaft 101 bends in a lengthwise direction of the vehicle as shown by a solid line. The long support span described above adds up the amount of such bending.
Generally, in the electric power steering apparatus of FIG. 8, there exists an error of meshing dimension between the rack 106 and pinion 107. Further, a clearance arises between screw grooves and balls of the ball nut mechanism 105. The rack shaft 101 thus encounters vibrations resulting from the causes as enumerated below:
(a) during running of the vehicle, especially when the steering wheel is turned, a road surface reactive force is transmitted to the rack shaft 101 via the tie rods 102, 102, thereby vibrating the rack shaft 101 in a front-and-rear direction of the vehicle (first cause); and PA1 (b) as each ball of the ball nut mechanism 105 comes into and out of contact with the respective screw groove, a force applied from the ball to the screw groove varies, thereby vibrating the rack shaft 101 (second cause). PA1 (1) to increase the accuracy of meshing dimension of the rack 106 and the pinion 107; PA1 (2) to reduce the clearance between the ball and the screw groove of the ball nut mechanism 105; and/or PA1 (3) to alter the diameter of the rack shaft 101 to thereby vary its natural frequency, or to provide the rack shaft 101 with a vibration restricting member.
When a frequency of the vibration resulted from the first cause and a frequency of the vibration resulted from the second cause match, the vibration of the rack shaft 101 is amplified. Further, when the frequency of the amplified vibration matches with the natural frequency of the rack shaft 101, the rack shaft 101 vibrates in a further increased magnitude due to resonance.
Such amplified vibration of the rack shaft 101 may be transmitted to the passenger compartment via the steering wheel, thereby producing noises in the passenger compartment. Transmission of the rack shaft vibration to the steering wheel is also undesired from the standpoint of a steering feel to the driver.
As measures to suppress the resonance of the rack shaft 101, one ay propose (1)-(3) as follows:
However, the measure (1) involves tedious processing, thereby increasing the costs of the resulting products. The measure (2) results in an increased frictional resistance between the ball and the screw groove and hence is undesired from the aspect of a steering feel. With the measure (3), the rack shaft 101 is increased in weight and complicated in construction.
Thus, it is demanded that an electric power steering apparatus be provided in which measures against the vibration of the rack shaft can be effected at low cost.